Drive Mechanism For A Drug Delivery Device

ABSTRACT

This invention relates to a drive mechanism for a drug delivery device, drug delivery devices comprising the drive mechanism of the invention and a method of assembling such drug delivery devices, which allow the user to select multiple doses of an injectable drug and to dispense the set dose of the drug.

This invention relates to a drive mechanism for a drug delivery devicethat allows the user to select multiple doses of an injectable drug andfor the dispensing of the set dosage of the drug and applying said drugto a patient, preferably by injection. In particular, the presentinvention relates to such devices, which are handled by the patientsthemselves.

BACKGROUND OF THE INVENTION

Drug delivery devices, which allow multiple dosing of the requireddosage of liquid drug and administration of the liquid to a patient, arewell known in the art. Generally, such devices have substantially thesame purpose as that of an ordinary syringe.

Injectors of this kind must meet a number of requirements to meet userneeds. These devices need to be robust in construction, yet easy to useboth in terms of the manipulation of the parts and understanding by auser of its operation. In the case of those with diabetes, many userswill be physically infirm and may also have impaired vision. Where theinjector is to be disposable rather than reusable, the injector shouldbe cheap to manufacture and easy to dispose of (preferably beingsuitable for recycling).

WO 9114467 A1 discloses a dispensing device, comprising a drivemechanism having a telescopic piston rod consisting of a first and asecond threaded member having equi-spaced threaded segments withnon-threaded segments therebetween. The device further comprises a dosesetting sleeve, which surrounds the second threaded member being coupledfor rotation and is threadedly engaged with the device shell. Thethreads of the three said components are of the same lead. The design ofthis device requires a body length to plunger length ratio of about 1:1in order to allow dispensing of relatively large doses. However, simpleand safe correction of a set overdose by the patient remains unresolvedwithout either dispensing the set amount of fluid or dismantling thecartridge.

WO 9938554 A2 teaches an injection syringe for apportioning set doses ofa medicine from a cartridge wherein a drive mechanism comprising aunidirectional coupling (ratchet) is disclosed which allows correctionof a set overdose without dispensing a set amount of fluid or requiringthe dismantling of the cartridge. The device is further designed so thatan initial reluctance has to be overcome before the unidirectionalcoupling can be rotated during dose dispensing.

WO 0195959 A1 discloses an injection device for injection of set dosesof medicine from a cartridge, in which syringe a dose is set by screwinga nut up along a threaded piston rod, whereby a dose setting drum, whichcarries on its cylindrical surface along a helix a scale of which anumber corresponding to the set dose is shown in a window in the housingof the syringe, and an injection button, which is elevated over the endof the syringe, are moved axially a distance which is larger than theaxial movement of the nut. A gear wheel transmission is establishedbetween the nut and the injection button such that the gearing makes themovement of the button larger and the force, which has to be exerted onthe injection button, is correspondingly reduced.

Accordingly, the problem to be solved by the instant invention is toreduce the actuation (injection) force required during drug dispensingespecially when dispensing large doses whilst maintaining a compactdesign and thereby improving user handling, and providing the user witha means to intuitively, safely and easily correct a set dose.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide analternative drive mechanism to existing technologies suitable for theefficient transmission of an actuation force in drug delivery devices,particularly compact devices, e.g., of pen-type shape, whichadditionally allows the dispensing of large doses of drug and whichfurther provides the advantage of intuitive and easy to use correctionof a set dose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 to 6 relate to the drug delivery device of embodiment 1, FIG. 7to 8 relate to the drug delivery device of embodiment 2 to which thepresent invention is applied.

FIG. 1 is a perspective view of the entire device.

FIG. 2 is a sectional side view showing the drive mechanism of thedevice in an initial state.

FIG. 3 is a sectional side view illustrating a dose setting operation ofthe device.

FIG. 4 is a sectional side view illustrating a dose delivery of thedevice.

FIG. 5 is an enlarged scale of FIG. 2.

FIG. 6 shows the plunger rod, the lead screw, and the plunger rod holderof the device.

FIG. 7 is a sectional side view showing the device in a clutched state.

FIG. 8 is a sectional side view showing the device in a declutchedstate.

FIG. 9 is a sectional side view showing an alternative of the free lock.

DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the instant invention is to provide a drive mechanismfor a drug delivery device comprising:

a plunger rod having a non-circular cross section and being hollow, saidplunger rod being prevented from rotation;

a lead screw engaged with said plunger rod;

a dose setting dial rotatably moving towards the proximal end duringdose setting and rotatably moving towards the distal end during dosedispensing;

an inner cylinder being releasibly connected to the dose setting dial,such that relative rotation between said dose setting dial and saidinner cylinder is prevented during dose setting but is allowed duringdose dispensing; and

a plunger rod holder, which prevents said plunger rod from movingtowards the proximal end during dose setting, but allows said plungerrod to move towards the distal end during dose dispensing;

optionally further comprising a free lock.

A second aspect of the instant invention is a drug delivery devicecomprising the drive mechanism according to the invention.

A third aspect of the instant invention is the use of a drive mechanismaccording to the invention in a method of assembling a device accordingto the invention.

A fourth aspect of the instant invention is the use of a drive mechanismor a device according to the invention for the administration of apharmaceutical formulation to the human or animal body.

Another aspect of the instant invention is a method of assembling a drugdelivery device according to the invention, comprising the step ofmounting the mechanism according to the invention to any components inorder to assemble a drug delivery device.

The terms, which are used in order to define the instant invention, aregenerally to be understood in accordance with the general knowledge ofthe person skilled in the art. Further, the following terms shall havethe following, optionally preferred, meaning in accordance with theinstant invention:

The term “drug delivery device” according to the instant invention shallmean a multi-dose, disposable, mobile, hand-held device designed todispense a selected dose of medicament, e.g. insulin, insulin analogues,growth hormones, low molecular heparins, and their derivatives etc.,optionally suitable for self-administration. Said device is of themechanical pen-type. Preferably, the term “drug delivery device” shallmean a disposable, multi-dose, pen-type device having mechanical dosedelivery and dose selection mechanisms designed for regular injection bypersons without formal medical training, e.g., patients. Usually, the“drug delivery device” of the instant invention comprises a cartridge,which contains a pharmaceutical formulation, which can be administeredvia a needle, and, optionally a cartridge holder.

The term “pharmaceutical formulation” shall preferably mean a liquid orsuspension or the like contained in the cartridge, comprising a drug orvaccine. The drug may contain one or more proteins, peptides, or smallmolecules, which may be administered subcutaneously. Preferably, thedrug is one or more hormone or antithrombotic, especially selected fromthe group consisting of insulins, heparins, derivatives, analogous, andsubstitutes thereof.

The term “housing” according to the instant invention shall preferablymean an exterior or interior (“insert”) cover. The housing may bedesigned to enable the safe, correct, and comfortable handling of thedrug delivery device (e.g., the drive mechanism). Usually, it isdesigned to house, fix, protect, guide, and/or engage with the innermechanism(s) or components of the drug delivery device (e.g., the drivemechanism) by limiting the exposure to contaminants, such as liquid,dust, dirt etc. In general, the housing may be unitary or a multipartcomponent of tubular or non-tubular shape. Preferably, the “housing” isdesigned to contain a cartridge and optionally a cartridge holder, whichis preferably mounted at the distal end of the housing.

The term “engaged” according to the instant invention shall mean theinterlocking of two or more components of the drive mechanism/drugdelivery device, preferably the interlocking of screw structures ofcomponents.

The term “screw structure” according to the instant invention shall meana full or part thread, e.g., a cylindrical spiral rib/groove located onthe internal and/or external surface of a component of the drug deliverydevice, having an essentially triangular or square or rounded sectiondesigned to allow continuous free rotational and/or axial movementbetween components. Optionally, a screw structure may be furtherdesigned to prevent rotational or axial movement of certain componentsin one direction. Screw structures are incorporated between the threadedinsert and the dose setting dial (hereinafter the “first screwstructure”), between the inner cylinder and the free lock (hereinafterthe “second screw structure”), and between the lead screw and theplunger rod (hereinafter the “third screw structure”). The said screwstructures according to instant invention shall preferably havedifferent screw pitches to allow force transmission from the proximalend to the distal end during dose delivery. Therefore, the screw pitchratios between the 1^(st), 2^(nd), and 3^(rd) screw structures are about1.8-4.2:1.8-4.2:1, preferably about 2.4-3.6:2.4-3.6:1.

It is further preferred, that the screw pitches of the 1^(st) screwstructure and the 2^(nd) screw structure are the same.

The term “dose setting dial” according to the instant invention shallpreferably mean an essentially tubular component of essentially circularcross-section having an external thread engaged with the housing bymeans of a first screw structure, allowing the dose setting dial torotatably move towards the proximal end during dose setting androtatably move towards the distal end during dose dispensing. The “dosesetting dial” according to the instant invention is designed to indicatea selected dose of the dispensable product (e.g., a drug). This may beachieved by use of markings, symbols, numerals, etc., e.g. printed onthe external surface of the dose setting dial or an odometer, or thelike.

Additionally, the dose setting dial is releasibly connected to the innercylinder by a clutch means. To set the dose, the dose setting dial isrotated and the dose setting dial and the inner cylinder rotate togethertowards the proximal end. During dose dispensing, the clutch meansdisengages the dose setting dial from the inner cylinder, such that thedose setting dial rotates relative to the inner cylinder towards thedistal end. Further, the dose setting dial may comprise one or morestops to limit the maximum amount of a single dose.

The term “inner cylinder” according to the instant invention shall meanany essentially tubular component of essentially circular cross-section,which is releasibly connected to the dose setting dial, such thatrelative rotation between said dose setting dial and said inner cylinderis prevented during dose setting but is allowed during dose dispensing.In a preferred embodiment the inner cylinder is further engaged with thelead screw by a free lock. In another preferred embodiment the innercylinder is further engaged with the piston rod in order to allow thelead screw to rotate with the inner cylinder during dose setting, e.g.,by means of splined projections located in a key way in the innersurface of the inner cylinder. During dose dispensing the inner cylinderis disengaged from the dose setting dial by a clutch means and is movedtowards the distal end without rotation (with respect to the housing).

The term “releasibly connected” according to the instant invention shallmean that two components of the instant mechanism or device arereversibly joined with each other, allowing coupling and decoupling.This is achieved, e.g., by a clutch means.

The term “plunger rod” according to the instant invention shall mean anycomponent adapted to operate through/within the housing, designed totransfer forces from the proximal end to the distal end of the drugdelivery device, preferably to the cartridge piston, for the purpose ofdrug dispensing. According to the instant invention, the “plunger rod”is essentially cylindrical, hollow, and has a non-circularcross-section. The “plunger rod” is a component of the drive mechanism,which is prevented from rotation with respect to the housing by means ofthe plunger rod holder. The “plunger rod” abuts the cartridge piston atthe distal end of the plunger rod. In a particular embodiment, the“plunger rod” has ratchet teeth or the like which interact with theplunger rod holder.

The term “plunger rod holder” according to the instant invention shallmean any component, which prevents movement of the plunger rod towardsthe proximal end during dose setting, but allows movement of the plungerrod towards the distal end during dose dispensing. Optionally, theplunger rod is also prevented from rotation by the plunger rod holder.In order to achieve the aforementioned, the plunger rod holder engageswith the plunger rod, e.g., by means of ratchet teeth and ratchet teetharms.

The plunger rod holder may be a separate component or be an integralpart of the housing or any other component. Furthermore, there are manyother suitable solutions within the knowledge of the person skilled inthe art in order to essentially prevent the plunger rod from movement inthe proximal direction.

The term “distal end” according to the instant invention shall mean theend of the device or a component of the device, which is closest to thedispensing end of the device.

The term “proximal end” according to the instant invention shall meanthe end of the device or a component of the device, which is furthestaway from the dispensing end of the device.

The term “clutch means” according to the instant invention shall meanany means, which releasibly connects the dose setting dial and the innercylinder and which is designed to allow rotation of the dose settingdial and the inner cylinder with respect to the housing when the dosesetting dial and the inner cylinder are coupled and, when both arede-coupled, allows rotation of the dose setting dial with respect to thehousing, but does not allow rotation of the inner cylinder with respectto the housing and allows axial movement of the inner cylinder.Accordingly, the term “clutch means” is any clutch engaging for thepurpose of reversibly locking two components in rotation, e.g., by useof axial forces to engage a set of face teeth (saw teeth, dog teeth,crown teeth) or any other suitable frictional faces.

The term “periphery” according to the instant invention shall usuallymean the surface of any part, preferably the surface along thelongitudinal axis.

The term “original position” according to the instant invention shallmean the starting position of the dose setting dial, i.e., when the setamount of dosage is zero (“00”). This is usually the case, when thedevice has not yet been used, a full cartridge has been loaded, or thedevice is in use and the set amount of drug has been completelyexpelled, respectively, dispensed.

The term “lead screw” according to the instant invention shall mean anyessentially cylindrical component, which is engaged with the plungerrod, preferably threadedly engaged, and rotates with respect to theplunger rod when moving towards the proximal direction during dosesetting, and moving axially towards the distal end without rotationduring dose dispensing. In a preferred embodiment, the lead screw isfurther engaged with a free lock, (e.g., a friction clutch, a thrustbearing or the like), preferably threadedly engaged.

The term “free lock” shall mean an essentially cylindrical componenthaving both an external and internal thread and being threadedly engagedwith the inner cylinder and threadedly engaged with the lead screw.Preferably, during dose setting and dose dispensing, relative rotationalmovement between a) the free lock and the inner cylinder, and b) thefree lock and the lead screw is allowed and relative axial movementbetween the free lock and the inner cylinder is allowed, whilst relativeaxial movement between the free lock and the lead screw is restricted.

In a more particular embodiment, the free lock is threadedly engagedbetween the inner surface of the inner cylinder and the external surfaceof the lead screw, optionally at the proximal end of the lead screw.

In yet another preferred embodiment, the free lock is fixed to theplunger rod holder, whereby relative axial movement between the freelock and the lead screw is not restricted.

Accordingly, the term “free lock” means a mechanism combining thecharacteristics of both a clutch mechanism (e.g., a slip clutch) and aforce reduction mechanism.

The term “counter ring” according to the instant invention shall meanany constructive element, which is in an interlocking relationship tothe dose setting dial. In a preferred embodiment, the counter ring isassembled concentrically on the outer circumference of the dose settingdial, optionally adjacent to the front side of the threaded insert. Inanother preferred embodiment, a positioning collar is assembled on theouter circumference of the dose setting dial adjacent to the front sideof the counter ring, optionally, the cylindrical positioning collar isintegrated into the housing.

The “counter ring” shall indicate one digit of the amount of the setdose by means of indices along its outer circumference, whereas one ormore further digits of the set dose is indicated by indices along theouter circumference of the dose setting dial. Preferably, dialing of the“counter ring” shall enable to set the smallest increments of the setdose (e.g., tens, eights, quarters, or halves of a unit, or singleunits).

The term “interlocking relationship” according to the instant inventionshall mean any constructive connection of the counter ring and the dosesetting dial, which allows both, the counter ring and the dose settingdial, to rotate together, preferably by means of a screw structure(e.g., thread, groove, rib), and also allows longitudinal axial movementof the dose setting dial with respect to the counter ring, when the dosesetting dial is moved (in either forward or backward direction).Preferably, the counter ring remains visible in the display window anddisplays the set dose (amount of drug), when the dose setting dial isscrewed out in order to set the dose.

The term “display window” according to the instant invention shall meanany opening in the housing, e.g. a hole, or a transparent section in thehousing, which allows displaying the status of the device, preferably,the status of dose setting, particularly, the amount of set dose. Thisis preferably achieved, e.g., by means of a dose indicator, whichexhibits one or more numerical or graphical symbols or values orcharacters, preferably two or three digits in order to indicate the setamount of dose. In another preferred embodiment, the “display window”displays the value of the selected amount of dose consisting of one ormore digits indicated along the circumference of the outer periphery ofthe counter ring, and one or more digits indicated along thecircumference of the outer periphery of the dose setting dial.

In another embodiment, the display window is preferably locatedessentially at the proximal end of the device.

According to the drug delivery device of the instant invention, thedistal end of the plunger rod abuts the cartridge piston and theproximal end of the plunger rod is engaged with the lead screw. Duringdose setting, the lead screw is driven to rotate by the inner cylindertowards the proximal end with respect to the plunger. The plunger rodand lead screw are configured in a telescopic-like structure, thereby,reducing the overall length of the drive mechanism.

Further, the plunger rod is engaged with the plunger rod holder, suchthat rotational movement during both dose setting and dose dispensing isprevented. The plunger rod holder is also designed to prevent linearmovement of the plunger rod towards the proximal end during dose settingbut allow distal linear movement of the plunger rod during dosedispensing. The mechanism of instant invention allows dose setting to beperformed repeatedly with great accuracy.

Proximal axial movement of the plunger rod during dose setting isprevented by the use of ratchet teeth formed along the outer peripheryof the plunger rod which engage with a plurality of ratchet teeth armsformed on the plunger rod holder.

Rotational movement of the non-circular cross section plunger rod isprevented by it being inserted in a corresponding non-circular passageway formed in the plunger rod holder.

Embodiment 1 of the present invention is described with reference to thedrawings.

An outline of the device (1) is described with reference to FIGS. 1 to2.

The device is of the mechanical pen type and consists of:

a housing (2);

a cartridge-holder (2 a) coupled to the distal end of the housing (2);

a cartridge (3), sealed by a piston (8) mounted in the proximal end ofthe cartridge and containing a liquid (4) drug, mounted in thecartridge-holder (2 a);

a removable cap (12) attached to the distal end of the device;

a piston driving assembly (21) assembled in the proximal end of thehousing (2).

The piston driving assembly transfers an activation force from theproximal end to the distal end during dispensing. The piston drivingassembly (21) consists of a dose setting dial (22), an inner cylinder(23), a free lock (24), a lead screw (25), a plunger rod (26), a plungerrod holder (27), a release knob (28), a dose indicator (or counter)(29), a counter ring (30) and a threaded insert (31).

Screw structures are incorporated between the threaded insert (31) andthe dose setting dial (22) (i.e., the “first screw structure”), betweenthe inner cylinder (23) and the free lock (24) (i.e., the “second screwstructure”), and between the lead screw (25) and the plunger rod (26)(i.e., the “third screw structure”).

Operation of the device will now be described. To set a desired dose tobe delivered a user rotates the rotating knob (22 a) of the dose settingdial (22), thus moving the dose setting dial towards the proximal end bymeans of the first screw structure (35). During dose setting relativerotation between the dose setting dial (22) and the inner cylinder (23)is prevented by a clutch (56), thus allowing the inner cylinder

When the inner cylinder (23) rotates towards the proximal end duringdose setting, the lead screw (25) rotates integrally with the innercylinder (23), with respect to the plunger rod (26), by virtue of a pairof splined projections (50) located in a key way (51) in the innersurface of the inner cylinder (23). When the lead screw (25) rotates outof the plunger rod (26) towards the proximal end, the plunger rod holder(27) locks the plunger rod (26) to prevent the plunger rod from beingdisplaced towards the proximal end thus maintaining the abutment withthe piston (8).

The rotation of the inner cylinder (23) further moves the free lock (24)towards the distal end through the second screw structure (44).

The set dose in the current embodiment is indicated by numerical values(66) on the indicator (counter) (29) and the dose setting dial (22)displayed in the display window (62).

To administer a selected dose to a patient, the user depresses therelease knob (28) towards the distal end, uncoupling the dose settingdial (22) from the inner cylinder (23) allowing the dose setting dial torotate relative to the inner cylinder and move towards the distal end.The inner cylinder (23) moves axially towards the distal end thusrotating the free lock (24) in a proximal direction at a predeterminedspeed-reducing ratio transferring the axial movement of the innercylinder (23) to the lead screw (25) moving it axially towards thedistal end. The lead screw (25) thus pushes the plunger rod (26) towardsthe distal end. This in turn pushes the piston (8) towards the distalend expelling the contents from the cartridge (3) through the needle (5)into the patient.

Details of the device of embodiment 1 are described with reference toFIGS. 2 to 6.

The threaded insert (31) is concentrically assembled and secured to theinside of the proximal end portion (2 b) of the housing (2) by suitablemethods known by those skilled in the art. The threaded insert (31) isthreadedly engaged with the essentially cylindrically shaped dosesetting dial (22) forming the first screw structure (35). The dosesetting dial (22) is mounted to allow rotational movement towards theproximal end during dose setting and the distal end during dosedelivery. Stops (22 d), preferably in the form of a pawl, are formed onthe distal end of the dose setting dial (22).

A cylindrical rotating knob (22 a) having a diameter equal to the outerdiameter of the housing is attached to the proximal end of the dosesetting dial (22). Serrations (22 b) are formed on the outer surface ofthe rotating knob (22 a) to improve grip for the user.

The inner cylinder (23) is assembled concentrically within the dosesetting dial (22) and is releasibly connected to the dose setting dial(22) by a clutch means (56). An essentially cylindrically shaped flangeportion is formed at the proximal end (23 a) of the inner cylinder (23)and inserted into the hollow recess (22 c) of the rotating knob (22 a).

The outer surface of the free lock (24) is threadedly engaged with theinner surface of the inner cylinder (23) forming the second screwstructure (44). The free lock (24) is free to rotate and move axiallytowards the distal end and the proximal end within the inner cylinder(23).

The inner surface of the free lock (24) is threadedly engaged with theouter surface of the proximal end of the essentially non-circular crosssection lead screw (25). The free lock (24) is free to rotate on theouter surface of the lead screw (25) and move axially towards the distalend and the proximal end of the lead screw (25).

The distal end of the lead screw (25) is threadedly engaged with theproximal end of the plunger rod (26) forming the third screw structure(47).

The essentially non-circular cross section, preferably a square tubeshape, plunger rod (26) has small pitch ratchet teeth (38) on its outersurface, preferably on two opposing faces of the outer surface.

The plunger rod holder (27) is secured to the inner side of the distalend of the housing (2) by any suitable means known by a person skilledin the art. A square passageway (37), having a dimension equal to theouter diameter of the plunger rod (26) is formed in the centre of theplunger rod holder (27). The plunger rod (26) is engaged with the squarepassageway (37), such that rotation of the plunger rod (26) is preventedby the plunger rod holder (27).

A plurality of pairs of flexible ratchet teeth arms (39), preferablytwo, is formed on the plunger rod holder (27). These form the ratchetmechanism (40) of the device. Further embodiments could be envisagedwhere three or four ratchet teeth arms are formed on the plunger rodholder (27) having, for example, 90° (e.g., 4 arms) or 120° (e.g., 3arms) offset, or the like.

The ratchet mechanism (40) is engaged with the ratchet teeth (38) of theplunger rod (26) such that axial movement of the plunger rod (26)towards the proximal end is prevented but axial movement of the plungerrod (26) towards the distal end is allowed.

A plurality of spline grooves (49), preferably two, is formed on theinner surface of the inner cylinder (23). A plurality of splinedprojections (50), preferably two, is formed on the outer surface of theproximal end of the lead screw (25). The splined projections (50) engagewith the spline grooves (49) of the inner cylinder (23) permitting axialmovement of the lead screw (25) towards the distal end and the proximalend with respect to the inner cylinder (23). The spline grooves (49) andthe splined projections (50) together form a splined structure (51).

A release knob (28) of essentially cylindrical shape, open at the distalend and closed at the proximal end, is assembled concentrically in thehollow (22 c) on the inner side of the rotating knob (22 a). An annularrib (53) is formed on the outer circumference of the release knob (28)and is engaged with an annular groove (52) on the inner circumference ofthe rotating knob (22 a). The release knob (28) is thus free to rotatesand move axially towards the distal end and the proximal end withrespect to the rotating knob (22 a). Movement of the release knob (28)towards the distal end and the proximal end is limited by the width ofthe annular groove (52).

The release knob (28) moves such that it can be brought in and out ofabutment with the flange portion (23 b) of the inner cylinder (23).

A wave-shaped annular (54), e.g., as teeth, or the like, is formed onthe distal end face of the flange portion (23 b) of the inner cylinder(23) and is engaged with a corresponding wave-shaped shoulder (55) onthe dose setting dial (22). The wave-shaped annular (54) and thewave-shaped shoulder (55) together form a first clutch (56) releasiblyconnecting the dose setting dial (22) and the inner cylinder (23) suchthat relative rotational movement is prevented during dose setting.

A second clutch (60) is formed between the proximal end of the innercylinder (23) and the inner face of the release knob (28). The secondclutch (60) consists of two complementary wave-shaped configurations,teeth-shaped configurations, or the like, located on the release knob(28) and the inner cylinder (23).

In the present embodiment the set dose is indicated as a numerical valueon the counter ring (30) and the dose setting dial (22). The set dose isviewed through a display window (62) located at the proximal end of thehousing (2). The display window (62) of the present embodiment is open,although a transparent lens could be used to cover the display window(62) to prevent the ingress of dirt, dust, liquid, etc., into themechanism and/or act as a magnifier.

The counter ring (30) is engaged with the dose setting dial (22) bymeans of a plurality of rib grooves (64) formed on the outercircumference of the dose setting dial (22) from its distal end to itsproximal end, and a corresponding number of ribs (63) formed on theinner surface of the counter ring (30). The counter ring (30) can moveaxially towards both the proximal end and distal end of the dose settingdial (22). To prevent axial movement of the counter ring (30) withrespect to the dose setting dial (22), a positioning collar (65) isassembled on the outer circumference of the dose setting dial (22) andsecured to the inner surface of the housing (2) adjacent to the proximalend of the counter ring (30). A cylindrical positioning collar (65),integrated into the housing (2), abuts the distal end of the counterring (30). Thus the counter ring (30) is able to rotate with the dosesetting dial (22) with respect to the housing during dose setting anddose delivery, but is prevented from axial movement with respect to thehousing (2).

In the present embodiment the counter ring (30) displays the “unit”values for the set dose from “0” to “9”. The “tens” values are indicatedon the dose setting dial (22) such that during on complete revolution ofthe counter ring (30) the corresponding “tens” value is continuouslydisplayed.

All three screw structures in the device according to instant invention(i.e., 35, 44, and 47) have differing screw pitches. The ratio in thescrew pitches between the first (35), the second (44), and the thirdscrew structure (47) is about 3:2:1, in order to achieve an efficientreduction of transmission forces (“speed reduction”) of the screwstructures.

Embodiment 2 of the device (1) is described with reference to FIGS. 7and 8.

The present embodiment 2 includes a modification to the first clutch(56), between the dose setting dial (22) and the inner cylinder (23).

FIG. 7 indicates an alternative clutch (57) that is located at theproximal end of the inner cylinder (23). The clutch (57) consists ofconical cam faces (57 a) and a plurality of equidistantly spaced firstengaging portions (54 a). Through the axial movement of the release knob(28) either towards the distal end or towards the proximal end, theclutch (57) can be compressed or expanded to cause the first engagingportions (54 a) to engage with or disengage from the correspondingsecond engaging portion (54 b) formed on the inner face of the rotatingknob (22 a). The first engaging portions (54 a) and the second engagingportion (54 b) are engaged, e.g., by the spring force of a pair ofspring acting portions (not shown).

During dose setting, the first engaging portions (54 a) and secondengaging portion (54 b) are engaged under the spring force thus couplingthe inner cylinder (23) with the dose setting dial (22) and preventingrelative rotation between the inner cylinder (23) and the dose settingdial (22).

During dose dispensing, the release knob (28) is pushed in the distaldirection disengaging the first engaging portions (54 a) from the secondengaging portion (54 b) thereby disconnecting the dose setting dial (22)from the inner cylinder (23) and allowing the dose setting dial (22) torotate with respect to the inner cylinder (23).

The mechanism and the device of the present invention is not limited tothe embodiments 1 and 2 described above but allows various effectivealterations based on the technical idea of the present invention. Forexample, FIG. 9 shows an alternative solution, wherein the free lock isfixed to the plunger rod holder.

In this case, the screw pitch ratios between the 1^(st), 2^(nd) and3^(rd) screw structures are about 3:3:1.

1. A drive mechanism for use in a drug delivery device comprising: a plunger rod having a non-circular cross section and being hollow, said plunger rod being prevented from rotation; a lead screw engaged with said plunger rod; a dose setting dial rotatably moving towards the proximal end during dose setting and rotatably moving towards the distal end during dose dispensing; an inner cylinder being releasibly connected to the dose setting dial, such that relative rotation between said dose setting dial and said inner cylinder is prevented during dose setting but is allowed during dose dispensing; and a plunger rod holder, which prevents said plunger rod from moving towards the proximal end during dose setting, but allows said plunger rod to move towards the distal end during dose dispensing.
 2. The drive mechanism according to claim 1 further comprising a free lock having both an external and internal thread and being threadedly engaged with the inner cylinder and threadedly engaged with the lead screw.
 3. The drive mechanism according to claim 1, wherein ratchet teeth are formed successively along the said plunger rod and one or more ratchet teeth arms are formed on said plunger rod holder for being successively engaged with said ratchet teeth for inhibiting movement of said plunger rod towards the proximal end.
 4. The drive mechanism according to claim 2, wherein ratchet teeth are formed successively along the said plunger rod and one or more ratchet teeth arms are formed on said plunger rod holder for being successively engaged with said ratchet teeth for inhibiting movement of said plunger rod towards the proximal end.
 5. A drug delivery device comprising the drive mechanism according to claim
 1. 6. A drug delivery device comprising the drive mechanism according to claim
 2. 7. A drug delivery device comprising the drive mechanism according to claim
 3. 8. A drug delivery device comprising the drive mechanism according to claim
 4. 9. A drug delivery device according to any one of claims 5, 6, 7 or 8 containing insulin, heparin, or a derivative or analogue thereof.
 10. A method of delivering a pharmaceutical formulation to a human or animal body comprising administering the pharmaceutical formulation from a drug delivery device comprising the drive mechanism according to claim
 1. 11. A method of delivering a pharmaceutical formulation to a human or animal body comprising administering the pharmaceutical formulation from a drug delivery device according to claim
 5. 12. A method according to claim 10 wherein the pharmaceutical formulation contains insulin, heparin, a derivative thereof or an analogue thereof.
 13. A method according to claim 11 wherein the pharmaceutical formulation contains insulin, heparin, a derivative thereof or an analogue thereof.
 14. A method of assembling a drug delivery device comprising a drive mechanism according to claim 1, comprising mounting the drive mechanism to any other components of the drug delivery device. 